1. Field of the Invention
This invention relates in general to magnetic read sensors, and more particularly to a method and apparatus for providing a free layer having higher saturation field capability and optimum sensitivity (ΔR/R).
2. Description of Related Art
The heart of a computer is typically a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with a parking ramp or the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. A device that is particularly well suited to the CPP design is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ is the change of resistivity of the tunnel junction between two ferromagnetic layers. When the magnetization of the two ferromagnetic layers is in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability.
A sensor includes a nonmagnetic electrically conductive or electrically nonconductive material spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer typically interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90. degree to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The sensor is located between ferromagnetic first and second shield layers. For a CPP sensor, first and second leads are connected to a bottom and a top respectively of the sensor for conducting a current perpendicular to the major thin film planes of the sensor. This is in contrast to a CIP sensor where the current is conducted in plane parallel to the major thin film planes (CIP) of the sensor.
A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
The sensitivity of the sensor is quantified as magnetoresistance or magnetoresistive coefficient ΔR/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. The sensitivity of a spin valve sensor depends upon the response of the free layer to signal fields from a rotating magnetic disk. The magnetic moment of the free layer or free layer structure depends upon the material or materials employed for the free layer structure. As the magnetic moment of the free layer structure increases the responsiveness of the free layer structure decreases. This means that for a given field signal from the rotating magnetic disk the magnetic moment of the free layer structure will not rotate as far from its parallel position to the ABS, which causes a reduction in signal output. The addition of a cobalt or cobalt based layer increases the stiffness of the free layer structure in its response to field signals and reduces the sensitivity of the spin valve sensor.
Although the layers enumerated above are all that is needed to produce the GMR or MTJ effects, additional problems remain. With longitudinal recording, the field from the media is stronger and thus an increase in the stiffness of the free layer would be desirable to prevent saturation. However, increasing the stiffness of the free layer structure in its response to field signals reduces the sensitivity of the spin valve sensor. Further, in order to meet higher signal requirements it would be desirable to reduce the thickness of the free layer besides improving the GMR ratio itself. However, thinning of the free layer causes a low GMR ratio and poor thermal stability. A synthetic free layer would seem to provide a way to maintain good thermal stability but, in both in CIP and CPP structures, synthetic free layers actually cause a GMR loss due to current shunting in CIP and the subtractive result of the free layers. It has been found that a free layer thickness of approximately 35 Å provides maximum sensitivity, i.e., ΔR/R. However, sensors with a free layer thickness of approximately 35 Å exhibit saturation for perpendicular reading environments.
It can be seen then that there is a need for a method and apparatus for providing a free layer having higher saturation field capability and optimum sensitivity (dr/R).